The hedgehog (Hh) gene was first identified during a search for embryonic lethal mutants of Drosophila melanogaster, which found that mutation of Hh resulted in altered segment patterning of the larva (Nusslein-Volhard, C.; Wieschaus, E. Nature 1980, 287, 795-801). Subsequently the gene was identified in many other invertebrates and vertebrates, including humans. Three mammalian counterparts of the Hh gene, termed Sonic hedgehog (Shh), Dessert hedgehog (Dhh), and cDNA libraries (Echelard, Y.; Epstein, D. J.; et al., Cell 1993, 75, 1417-1430). Hh undergoes multiple processing events, including autocatalytic cleavage of the C-terminal domain combined with addition of a cholesterol moiety at the cleavage site, and an N-terminal palmitoylation, to generate the active ligand (Lee, J. J.; Ekker, S. C.; et al., Science 1994, 266, 1528-1537; Porter, J. A.; Young, K. E.; et al., Science 1996, 274, 255-259; Pepinsky, R. B.; Zeng, C.; et al., J. Biol. Chem. 1998, 273, 14037-14045).
The receptor of secreted Hh protein is the multipass transmembrane protein Patched (Ptch). Of the two vertebrate homologues of Ptch (Ptch1 and Ptch2), the role of Ptch1 is better understood. In the absence of Hh ligand, Ptch inhibits the activity of the downstream effector Smoothened (Smo). The binding of Hh inactivates Ptch, resulting in activation of Smo (Stone, D. M.; Hynes, M.; et al., Nature 1996, 384, 129-134). In Drosophila, a complex of proteins comprising Fused (Fu), Suppressor of Fused (SuFu), and Costal-2 (Cos2) mediates signaling downstream of Smo and is aided by several kinases, such as protein kinase A (PKA), glycogen synthase kinase 3 (GSK3), and casein kinase 1 (CK1). Mammalian homologues of Fu and Cos2 have not yet been identified, suggesting that the signaling mechanisms differ in mammals and Drosophila. Several mammalian-specific kinases that is required for Shh signaling have been identified (Varjosalo, M.; Bjorklund, M.; et al., Cell 2008, 133, 537-548; Mao, J.; Maye, P.; et al., J. Biol. Chem. 2002, 277, 35156-35161; Riobo, N. A.; Haines, G. M.; et al., Cancer Res. 2006, 66, 839-845). These proteins modulate the function of Gli (Ci in Drosophila), the only transcription factor identified to date that operates directly downstream of Hh.
The first vertebrate Gli gene to be discovered was human Gli1, which was amplified about 50-fold in a malignant glioma (Kinzler, K. W.; Bigner, S. H.; et al., Science 1987, 236, 70-73). Vertebrates have three Gli proteins (Gli1, Gli2, and Gli3), all of which have five highly conserved tandem zinc fingers, a fairly conserved N-terminal domain, several potential PKA sites, and a number of additional small conserved regions in the C-terminal end. Despite these similarities, the functions of the Gli subtypes differ. Both Gli2 and Gli3 contain activation and repressor domains. Consequently, in the absence of upstream Hh signal, full-length Gli3 and, to a lesser extent, Gli2 are constitutively cleaved to generate a truncated repressor form (Dai, P.; Akimaru, H.; et al., J. Biol. Chem. 1999, 274, 8143-8152; Ruiz i Altaba, DeVelopment 1999, 126, 3205-3216; Shin, S. H.; Kogerman, P.; et al., Proc. Natl. Acad. Sci. U.S.A. 1999, 96, 2880-2884). Hh signaling inhibits this cleavage, resulting in full-length Gli2 and Gli3, which have activator function. Gli1, in contrast, does not undergo proteolytic cleavage and acts as a constitutive activator. The transcription of Gli1 gene is initiated by Hh and is also controlled by Gli3.27. Target genes of the Hh pathway other than Gli1 include Ptch, several Wnt and TGF superfamily proteins, cell cycle proteins such as cyclin D, and stem-cell marker genes such as NANOG and SOX2.30,31. Investigators are now attempting to comprehensively identify the Gli1-target genes (Yoon, J. W.; Kita, Y.; et al., J. Biol. Chem. 2002, 277, 5548-5555; Yoon, J. W.; Gilbertson, R.; Mt. J. Cancer 2008, 124, 109-119).
The Hh signaling pathway is crucial for proper embryonic development (Ingham, P. W.; McMahon, A. P. Genes Dev. 2001, 15, 3059-3087). It is also essential for restraining growth in the nervous system and other tissues and in maintenance of stem cells in adults (Machold, R.; Hayashi, S., et al., Neuron 2003, 39, 937-950; Lavine, K. J.; Kovacs, A.; et al., J. Clin. InVest. 2008, 118, 2404-2414. Balordi, F.; Fishell, G. et al., J. Neurosci. 2007, 27, 14248-14259). The expression and roles of Hh in vertebrate tissues/organs have been extensively described in the recent reviews (Varjosalo, M.; Taipale, J. Genes DeV. 2008, 22, 2454-2472).
Two of the functions of Hh in vertebrate embryonic development are both crucial and relatively well understood: neural tube differentiation and anteroposterior limb patterning. The predominant mechanism of Hh signaling in these functions is paracrine signaling, in which the Hh molecules act in a gradient fashion. For example, in vertebrate limb buds, exposure to different concentrations of Shh modulates patterning of the interdigital mesenchyme, which influences the proper growth of digits in a specific pattern (Tabin, C. J.; McMahon, A. P. Science 2008, 321, 350-352). In neural tube development, Shh produced by the floor plate causes dorsoventral patterning, the specification of ventral cell populations, and general cellular proliferation in the brain. 40 Holoprosencephaly, a disorder involving the development of forebrain and midface in which ventral cell types are lost, is caused in humans by mutations that lead to loss of Shh activity (Belloni, E.; Muenke, M.; et al., Nat. Genet. 1996, 14, 353-356).
Another important feature of Shh signaling is that the Gli subtypes have both unique and overlapping functions. While ectopic expression of Gli1 in the midbrain and hindbrain of transgenic mice results in expression of some ventral cell types, mice homozygous for a mutation in the region encoding the zinc finger domain of Gli1 develop normally (Hynes, M.; Stone, D. M.; et al., Neuron, 1997, 19, 15-26; Park, H. L.; Bai, C.; et al., Development 2000, 127, 1593-1605). However, Gli1/Gli2 double mutant mice have phenotypes with severe multiple defects, including variable loss of the ventral spinal cord, and smaller lungs; therefore, Gli2 plays a more important role in spinal cord and lung development than does Gli1. In contrast, Gli1/Gli3 double mutant mice did not have these phenotypes (Park, H. L.; Bai, C.; et al., DeVelopment 2000, 127, 1593-1605). Gli2 and Gli3 have both been implicated in skeletal development, with each subtype serving specific functional roles. Gli2 mutant mice exhibit severe skeletal abnormalities including cleft palate, tooth defects, absence of vertebral body and intervertebral discs, and shortened limb and sternum (Mo, R.; Freer, A. M.; et al., DeVelopment 1997, 124, 113-123). Gli3 appears to be the major mediator of Shh effect in the limbs, as Gli1/Gli2 double mutant mice had a normal digit number and pattern while Gli3 mutant mice showed polydactyly (Hui, C. C.; Joyner, A. L. Nat. Genet. 1993, 3, 241-246).
Genetic analyses of Gli mutants revealed that the requirement for Gli subtypes development is quite divergent even among vertebrates. In zebrafish, both detour (dtr) mutations (encoding loss-of-function alleles of Gli1) and you-too (yot) mutations (encoding C-terminally truncated Gli2) have defects in body axis formation and expression of Hh-target genes in the brain (Karlstrom, R. O.; Tyurina, O. V.; et al., DeVelopment 2003, 130, 1549-1564), suggesting divergent requirements for Gli1 and Gli2 in mouse and zebrafish.
In adults, the Hh pathway is essential for restraining growth in the nervous system and other tissues and in maintenance of stem cells. Zhang and Kalderon have shown that Hh acts specifically on stem cells in Drosophila ovaries and that these cells cannot proliferate in the absence of Hh (Zhang, Y.; Kalderon, D. Nature 2001, 410, 599-604). Other studies showed that Hh signaling in the postnatal telencephalon both promotes proliferation and maintains populations of neural progenitors, suggesting that Shh signaling in the mammalian telencephalon may participate in the maintenance of a neural stem cell niche. The role of Hh in proliferation of adult neural progenitor cells was confirmed by a study in which Shh was overexpressed and proliferation was inhibited by using a Smo antagonist (Lai, K.; Kaspar, B. K.; et al., Nat. Neurosci. 2003, 6, 21-27).
Hh genes have the ability to induce tissue proliferation. This function is important in embryogenesis and tissue maintenance, but inappropriate activation of the pathway can result in tumorigenesis (Hunter, T. Cell 1997, 88, 333-346). Tumors in about 25% of all cancer deaths are estimated to involve aberrant Hh pathway activation. Tumorigenesis or tumor growth can result from abnormal up-regulation of Hh ligand or from deregulation of the expression or function of downstream components by, for example, loss of Ptch, activating mutations of Smo (Xie, J.; Murone, M.; et al., Nature 1998, 391, 90-92), loss of SuFu, amplification or chromosomal translocation of Gli1 or Gli2 gene amplification or stabilization of Gli2 protein (Bhatia, N.; Thiyagarajan, S.; J. Biol. Chem. 2006, 281, 19320-19326).
The first Hh pathway gene found to be amplified in cancers was Gli1, which was expressed at high levels in human glioblastoma and derived cell lines. Subsequently, Gli1 was found to be consistently expressed in a variety of glial tumors, and Gli1 overexpression was shown to induce central-nerves system hyperproliferation (Dahmane, N.; Sanchez, P.; et al., Development 2001, 128, 5201-5212). Gli1 overexpression has also been observed in a panel of brain tumors ranging from low-grade to high-grade in a study that identified Gill expression as the only reliable marker of Hh pathway activity (Clement, V.; Sanchez, P.; Curr. Biol. 2007, 17, 165-172). Further, cell proliferation in primary cultures of many of these tumors was inhibited by Gli1 small-interfering RNA. Gli1 expression was correlated with tumor grade in PDGF-induced liomagenesis in mice. Hh signaling components other than Gli1 also contribute to tumorigenesis in specific subsets of glioblastomas. In PDGF induced tumors, expression level of Shh was correlated with the tumor grade. However, other studies found only a subset of gliomas to contain high levels of Shh.
Another cancer with defects in Hh pathway regulation is basal cell carcinoma (BCC). Human Ptch was first identified by virtue of its mutation in patients with Gorlin syndrome (GS), a genetic disease that gives rise to sporadic BCC (Johnson, R. L.; Rothman, A. L.; et al., Science 1996, 272, 1668-1671). The mutations of Ptch identified in BCC include deletions producing truncated proteins and insertion or nonsense mutations accompanied by loss of heterozygosity (LOH) or mutations in the other allele. These mutations inhibit the ability of Ptch to suppress Smo, resulting in constitutive Hh signaling. While Ptch1 abnormalities are detected in the majority of BCC patients, it is now clear that a subset of BCC is also driven by a mutation in Smo that decreases its sensitivity to inhibition by Ptch. In addition, overexpression of Gli1 protein causes BCC-like tumors in mice, establishing the importance of Gli1 transcription in BCC tumorigenesis (Nilsson, M.; Unden, A. B.; et al, Proc. Natl. Acad. Sci. U.S.A. 2000, 97, 3438-3443). The level of Gli1 transcript can be used to discriminate BCC from certain other skin tumors (Hatta, N.; Hirano, T.; et al., J. Cutaneous Pathol. 2005, 32, 131-136). However, blocking of Gli-based transcription has not yet been shown to arrest BCC growth.
Medulloblastoma, the most common malignant pediatric brain tumor, is linked with mutations in Ptch and Smo and mutations in other Hh pathway genes such as SuFu and Gli (Pomeroy, S. L.; Tamayo, P.; et al., Nature 2002, 415, 436442). Inactivation of the Ptch locus by deletion and mutation has been found in about 10% of sporadic medulloblastomas. Shh pathway involvement in these tumors was further confirmed by studies in which treatment of murine medulloblastomas with Smo inhibitors inhibited cell proliferation and reduced tumor growth in mice (Berman, D M.; Karhadkar, S. S.; et al., Science 2002, 297, 1559-1561; Sanchez, P.; Ruiz i Altaba, Mech. DeV. 2005, 122, 223-230; Romer, J. T.; Kimura, H. et al., Cancer Cell 2004, 6, 229-240). Taylor et al. identified SuFu as a tumorsuppressor gene whose mutation predisposes individuals to medulloblastoma. They found that a subset of children with medulloblastoma carry germline and somatic mutations in SuFu, accompanied by loss of heterozygosity of the wild-type allele. Several of these mutations encoded truncated SuFu proteins that are unable to export Gli protein from the nuclei. In addition, the tumor-suppressor REN has also been linked with medulloblastoma in which the allelic deletion and reduced expression of REN are frequently observed. It is suggested that it inhibits medulloblastoma growth by negatively regulating the Hh pathway (C.; Zazzeroni, F.; Gallo, R.; et al., Proc. Natl. Acad. Sci. U.S.A. 2004, 101, 10833-10838; Argenti, B.; Gallo, R.; et al., J. Neurosci. 2005, 25, 8338-8346).
Hh has also been shown to be an early and late mediator of pancreatic cancer tumorigenesis. Shh was not detected in normal adult human pancreata but was aberrantly expressed in 70% of pancreatic adenocarcinoma specimens (Thayer, S. P.; di Magliano, M. P.; et al., Nature 2003, 425, 851-856). Participation of Shh signaling has been indicated at multiple stages of pancreatic carcinogenesis and is accompanied by multiple oncogenic factors, including K-Ras, one of the most frequently mutated genes in pancreatic cancer (Morton, J. P.; Mongeau, M. E.; et al., Proc. Natl. Acad. Sci. U.S.A. 2007, 104, 5103-5108; Ji, Z.; Mei, F. C.; et al., J. Biol. Chem. 2007, 282, 14048-14055). Activated Hh signaling was detected in cell lines established from primary and metastatic pancreatic adenocarcinomas, and the Smo inhibitor cyclopamine induced apoptosis in a subset of the pancreatic cancer cell lines both in culture and in mice (Sheng, T.; Li, C.; et al., Mol. Cancer. 2004, 3, 29).
Numerous studies indicate that Hh signaling is involved in prostate cancer. Sanchez and others reported the expression of Shh-Gli pathway components in adult human prostate cancer. Treatment of primary prostate tumor cultures and metastatic prostate cancer cell lines with Smo inhibitors blocked the pathway and proliferation. Increased expression of Shh in prostate cancer cells up-regulates Gill expression and dramatically accelerates the growth of prostate tumor xenografts (Fan, L.; Pepicelli, C. V.; et al., Endocrinology 2004, 145, 3961-3970). Elevated Shh activity distinguished metastatic from localized prostate cancer, and manipulation of this pathway modulated the invasiveness and metastasis of these tumors (Karhadkar, S. S.; Bova, G. S.; et al., Nature 2004, 431, 707-712).
Hh signaling has also been implicated in various other cancers, such as lung, colorectal, bladder, endometrial, ovarian, and esophageal carcinomas and rhabdomyosarcoma (Chi, S.; Huang, S.; et al., Cancer Lett. 2006, 244, 53-60; Watkins, D. N.; Berman, D. M.; et al., Nature 2003, 422, 313-317; Qualtrough, D.; Buda, A.; et al., Mt. J. Cancer 2004, 110, 831-837; McGarvey, T. W.; Maruta, Y.; Oncogene 1998, 17, 1167-1172; Feng, Y. Z.; Shiozawa, T.; et al., Clin. Cancer Res. 2007, 13, 1389-1398; Bhattacharya, R.; Kwon, J.; et al., Clin. Cancer Res. 2008, 14, 7659-7666; Mori, Y.; Okumura, T.; et al., Oncology 2006, 70, 378-389; Tostar, U.; Malm, C. J.; et al., J. Pathol. 2006, 208, 17-25; Hahn, H.; Wojnowski, L.; et al., Nat. Med. 1998, 4, 619-622). The role of Hh-Gli signaling pathway in cancer and its potential as therapeutic target have been reviewed in more detail in recent articles.
The aberrant activation of Hh-Gli signaling in several cancers has made it an attractive target for anticancer drug discovery. Various inhibitors of hedgehog signaling have been investigated such as cyclopamine, a natural alkaloid that had been showed to arrest cell cycle at arrest cell cycle at G0-G1 and to induce apoptosis in SCLC. Cyclopamine is believed to inhibit Smo by binding to its heptahelical bundle. Currently cyclopamine is in preclinical and clinical studies as an anticancer agent (Kolterud, A.; Toftga°rd, R. Drug Discovery Today: Ther. Strategies 2007, 4, 229-235). A number of Smo inhibitors have now been reported and can be classified as cyclopamine analogues or synthetic Smo antagonists. Several pharmaceutical companies have identified new Smo inhibitors with druglike properties by optimization of high throughput screen hits.
One such small molecule, GDC-0449, was developed by Curis and Genentech, is currently in phase I/II clinical trials for advanced BCC and solid epithelial tumor (Gunzner, J.; Sutherlin, D.; et al., WO2006028958, Mar. 16, 2006). Despite with these compounds, there still remains a need for potent inhibitors of the hedgehog signaling pathway.